Field of the Invention
The present invention generally relates to optical amplifiers, and in particular it relates to an optical amplifying element, as well as light source devices and image pickup devices equipped with such optical amplifying element.
Description of Related Art
Wavelength tunable lasers capable of changing oscillation wavelengths are applied to Optical Coherence Tomography (OCT) apparatuses, and active research and development in this type of lasers has been conducted to improve efficiency and other parameters related thereto. Known wavelength tunable lasers include a laser that controls the oscillation wavelength of a vertical-cavity surface-emitting laser (generally referred to as VCSEL) by a Micro Electro Mechanical Systems (MEMS) technique, so-called MEMS-VCSEL. Specifically, the MEMS-VCSEL varies a cavity length by mechanically moving one of a pair of reflectors to change the laser oscillation wavelength.
Since output of the MEMS-VCSEL is small, in order to use the MEMS-VCSEL for a light source device, the light source device is configured such that an optical amplifying element amplifies the light output of the MEMS-VCSEL. In an article entitled “OCT Imaging up to 760 kHz Axial Scan Rate Using Single-Mode 1310 nm MEMS-Tunable VCSELs with >100 nm Tuning Range”, by V. Jayaraman, et al., disclosed in the Lasers and Electro-Optics (CLEO) Conference, 1-2 (2011), OCT image pickup is performed by using a light source device that uses the MEMS-VCSEL and a Semiconductor Optical Amplifier (hereinafter referred to as SOA). The SOA amplifies light emitted from the MEMS-VCSEL.
When intensity of a laser beam is amplified using SOA described in V. Jayaraman, et al, “OCT Imaging up to 760 kHz Axial Scan Rate Using Single-Mode 1310 nm MEMS-Tunable VCSELs with >100 nm Tuning Range”, Lasers and Electro-Optics (CLEO) Conference, 1-2 (2011), light called amplified spontaneous emission (hereinafter referred to as ASE) is also output, in addition to the amplified laser beam. The ASE is light obtained by amplifying light generated by spontaneous emission within the SOA by stimulated amplification. Accordingly, the ASE has no coherency with the laser beam amplified by the SOA.
Conventionally, in order to increase a signal-to-noise ratio (SN ratio), intensity of the laser beam is preferably increased; however, in an opthalmological OCT apparatus for acquiring a tomogram of a fundus of an eye, the intensity of the laser beam to be projected on a subject's eye is limited due to safety concerns. Since the ASE is contained in the light to project on a measurement object together with the laser beam, in order to limit optical output, it is necessary to limit the intensity of the beam including intensity of the ASE to equal to or less than a limiting value. Meanwhile, since the OCT uses interference of the laser beam, the ASE that does not interfere with the laser beam cannot be a signal for obtaining the tomogram by OCT. That is, if the ASE exists in an OCT apparatus, it is necessary to lower the intensity of the laser beam that generates an interfering signal compared with a case where the ASE does not exist, resulting in lower SN ratio.